Created attachment 83890[details]
dmesg from UX31A running under 3.10.5 with drm.debug=0xe
This has been reported in ALT Linux bugzilla (in Russian), posting here at Kirill's request as I've confirmed a report by another user on the same hardware I happen to use.
3.10.x kernels up to and including 3.10.5 result in turning off the backlight at i915 module load time on ASUS UX31A (i7-3517U); dmesg with drm.debug=0xe and late module load (not in initrd) is attached; external VGA monitor works fine (dmesg and Xorg.1.log available if needed).
Here's a transcript of sysfs knob fiddling over ssh:
root@ux class/backlight/intel_backlight # cat actual_brightness
0
root@ux class/backlight/intel_backlight # cat max_brightness
4302
root@ux class/backlight/intel_backlight # echo 800 > brightness
root@ux class/backlight/intel_backlight # cat actual_brightness
4302
and corresponding bits of dmesg (including further attempts at putting 100 and 80 values into brightness; both resulted in actual_brightness reading the same values but the screen backlight is still off):
[ 877.052934] [drm:intel_panel_actually_set_backlight], set backlight PWM = 4302
[ 881.868818] [drm:intel_panel_get_backlight], get backlight PWM = 4302
[ 974.670199] [drm:intel_panel_get_backlight], get backlight PWM = 4302
[ 978.469350] [drm:intel_panel_actually_set_backlight], set backlight PWM = 100
[ 981.176898] [drm:intel_panel_get_backlight], get backlight PWM = 100
[ 990.186065] [drm:intel_panel_actually_set_backlight], set backlight PWM = 80
acpi_video0 doesn't have any effect either.
This bug hasn't manifested itself on 3.9.7 with very similar configuration on the same hardware.
I've skimmed over bug #51394 and bug #66462 which might be related; #54687 and #65652 seem rather not.

(In reply to comment #2)
> could you please verify any of http://en.altlinux.org/Regular#main
> live images which happen to use the same 3.10.5 by now?
Maybe this helps: today's snapshot including 3.10.6 just works on my UX31A in both BIOS and UEFI modes *if* CSM is enabled in firmware; disabling Boot > Launch CSM triggers the bug.
The images are UEFI/BIOS, CD/Flash hybrid (just dd it onto a stick).

(In reply to comment #9)
> please attach /sys/kernel/debug/dri/0/i915_opregion for non-working
> and working setting of the BIOS option
OK, I'll have to get to the office so another console with ssh is handy.